Find bpy function in source code
Get the idname like:

bpy.ops.render.render.idname()
# 'RENDER_OT_render'


(or simply take render.render and replace the . by _OT_)

Search that operator in the source code, and find it here:

blender\source\blender\editors\render\render_internal.c




/* contextual render, using current scene, view3d? */
void RENDER_OT_render(wmOperatorType *ot)
{
    PropertyRNA *prop;

    /* identifiers */
    ot-&gt;name = "Render";
    ot-&gt;description = "Render active scene";
    ot-&gt;idname = "RENDER_OT_render";

    /* api callbacks */
    ot-&gt;invoke = screen_render_invoke;
    ot-&gt;modal = screen_render_modal;
    ot-&gt;cancel = screen_render_cancel;
    ot-&gt;exec = screen_render_exec;


You'll likely wanna look for the exec function code (screen_render_exec) and see what it does.